1. Field of the Invention
The present invention relates to a panel display for displaying data by utilizing a memory function for display cells selected from among a plurality of display cells formed at the intersections of a plurality of electrodes arranged in the form of a matrix, and to a driving method for the panel display. More particularly, this invention is concerned with a driving method for a plasma display panel (hereinafter, a PDP) or the like and a panel display adopting the driving method. In particular, the present invention relates to a panel display in which display glowing periods within sub-frames are weighted differently and thus differentiated from one another in order to achieve gray-scale display, and a driving method for the panel display.
In recent years, there have been increasing demands for thinness, diversification of information to be displayed and of conditions for installation, large screens, and high definition in the field of displays. The advent of a display meeting these demands has been awaited. A thin display falls into several types represented by an LCD, fluorescent character display tube, EL, and PDP respectively. In such a thin display, for gray-scale display, one frame is generally composed of a plurality of sub-frames, the sub-frames are weighted differently and thus differentiated from one another, and bits of gray-scale data are displayed during associated sub-frames.
A display using a PDP is attracting attention owing to its superb features such as no flickering, ease in making a screen larger, high luminance, and long service life. Herein, the description will proceed by taking an AC-type PDP for instance. The present invention can apply to the above types of displays.
As for display panels for a picture display available these days, there is a trend toward a larger picture receptor that can be adapted to television conformable to high-definition broadcasting standards such as HDTV and EDTV-II or the wide television conformable to a prevailing National Television System Committee (NTST)-recommended system. However, an existing cathode-ray tube type picture receptor has a limit in size. The color display PDP is therefore predicted to be used as a next-generation television.
The AC-type PDP is such that a voltage is applied alternately to two kinds of sustaining electrodes in order to sustain discharge, and thus a glowing display is achieved. One discharge is completed within one microsecond to several microseconds after application of a pulse. Ions that are positive charges induced by the discharge are accumulated on the surface of an insulating layer over one kind of electrode to which a negative voltage has been applied. Likewise, electrons that are negative charges are accumulated on an insulating layer over the other kind of electrode to which a positive voltage has been applied.
First, cells are discharged by applying a pulse (writing pulse) of a high voltage (writing voltage), and wall charges are produced. Thereafter, a pulse (sustaining pulse) of an opposite polarity and a lower voltage (sustaining voltage) is applied. The previously-accumulated wall charges are then superposed on the sustaining voltage. This results in a high voltage relative to a discharge space. Consequently, a threshold discharge voltage is exceeded and discharge is started. In the cells each having a wall charge produced after one writing discharge, discharge is sustained by applying a sustaining discharge pulse alternately. The feature of the AC-type PDP attributable to the nature of a cell is referred to as a memory effect or a memory function. In general, the AC-type PDP uses the memory function for displaying.
A PDP cannot vary its glowing strength. A luminance is substantially varied by differentiating the lengths of glowing periods from one another, whereby a gray-scale display is achieved. A gray-scale display by the PDP is usually achieved by varying the lengths of glowing periods associated with weights applied to bits of display data. A period of glowing associated with each bit is referred to as a sub-frame. This display system is referred to as a sub-frame system. For example, when 256-level gray-scale display is performed, display data is represented by eight bits. Display of one frame is divided into those of eight sub-frames. Display of each bit data is performed during each sub-frame. The ratio of the lengths of sub-frames is 1:2:4:8:16:32:64:128. One sub-frame is divided into a reset period, addressing period, and sustaining discharge period. During the reset period, a full-screen writing pulse is applied for self-erasing discharge, and all cells in a panel enter a uniform state without any wall charge. Next, during the addressing period, addressing discharge is performed line-sequentially in order to turn on or off the cells according to display data, and a wall charge permitting sustaining discharge is accumulated on cells to be enabled to glow. Thereafter, when the sustaining discharge period is completed, a picture of one sub-frame is displayed. In such an "addressing/sustaining discharge-separated type writing addressing system," a luminance is determined with the length of a sustaining discharge period; that is, the number of sustaining pulses.
In a known AC-type PDP, a plurality of display cells constituting one line are driven by pairs of a common sustaining electrode and a scan electrode. In this case, a current used to show display data on each line is substantially proportional to a quantity of display data (load) in the display cells. Resistive components are distributed within each electrode. The longer an electrode becomes, the larger the resistance of the electrode is. When a display current is supplied, the resistive components in an electrode bring about a voltage drop. The magnitude of a voltage drop is dependent on the quantity of display data. Furthermore, a floating capacitance is originally present between electrodes. Due to the floating capacitance, unnecessary charges are accumulated. This also leads to a voltage drop. The sustaining electrode and the scan electrode are led out alternately in opposite directions. The display luminances of display cells constituting the same line are therefore even. Consequently, a difference in luminance seldom occurs between display cells on the same line irrespective of whether a quantity of display data is larger or small. However, there is a difference in voltage drop between lines according to a quantity of display data on each line. This leads to a difference in display luminance between lines.
The foregoing voltage drop, dependent on a quantity of display data, occurs between sub-frames. That is to say, a sustaining pulse is applied to a sustaining electrode and a scan electrode by a drive circuit referred to as a sustainer. A current supplied by the drive circuit therefore varies depending on the number of cells to be discharged and sustained during each sub-frame; that is, a display load. Consequently, luminances to be attained during sub-frames do not demonstrate a given ratio but fluctuate.
The effective brightness of display during each sub-frame is determined with a luminance attained by sustaining discharge and a length of a sustaining discharge period. The sustaining discharge periods within sub-frames are set according to a given ratio. When the display loads to be imposed during the sub-frames are the same, luminances attained by sustaining discharge are the same. The brightnesses of display attained during the sub-frames demonstrate the same ratio as the sustaining discharge periods within the sub-frames. However, when the display loads to be imposed during the sub-frames are different from one another, the luminance attained by sustaining discharge differs among the sub-frames. The brightnesses of display attained during the sub-frames do not therefore demonstrate the given ratio. When this event occurs, gray scale rendered by combining the sub-frames cannot be displayed precisely. At worst, there arises a problem that brightnesses may be inverted between gray levels.